(defun days-in-month (month year)
  (case month
    (9 2)
    (4 2)
    (6 2)
    (11 2)
    (2 (if (= 0 year) 1 0))
    (otherwise 3)))

(defun sundays ()
  (do* ((i 0 (1+ i))
        (d 1 (let ((month (mod i 12))
                   (year (mod (floor (/ i 12)) 4)))
               (mod (+ d (days-in-month month year)) 7)))
        (total 0 (if (= 0 d) (1+ total) total)))
      ((>= i 1200) total)))